Method and stencil for extruding material on a substrate

ABSTRACT

A stencil for use in fabricating semiconductor devices is disclosed that has an aperture having a first portion extending from a first side thereof and a second portion extending from a second side thereof to minimize the shear stress between the material extruded therethrough and the stencil. The stencil allows for material to be extruded through the top of the stencil to the surface of the substrate and not contact the wall of the second portion of the aperture of the stencil. Since the material only contacts a small area of the first portion of the aperture near the top of the stencil, the material remains on the substrate and not in the aperture of the stencil.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/155,654,filed May 23, 2002, now U.S. Pat. No. 6,584,897, issued Jul. 1, 2003,which is a continuation of application Ser. No. 09/894,935, filed Jun.28, 2001, now U.S. Pat. No. 6,427,587, issued Aug. 6, 2002, which is acontinuation of application Ser. No. 09/572,738, filed May 17, 2000, nowU.S. Pat. No. 6,269,742 B1, issued Aug. 7, 2001, which is a continuationof application Ser. No. 09/030,047, filed Feb. 24, 1998, now U.S. Pat.No. 6,089,151, issued Jul. 18, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to applying a predeterminedvolume of material to a predetermined location on a substrate and, moreparticularly, to the use of a stencil having apertures that minimize theshear stress of a material applied using the stencil to a substrate.

2. State of the Art

The use of screen printing stencils is well known in the art. Screenprinting stencils are used in a wide variety of applications in theelectronic substrate fabrication and electronic assembly industry forapplying materials such as photo resist or solder paste.

As the size of the features of a semiconductor device continues todecrease with each generation, ever greater precision is required inorder to apply viscous material to the surface thereof. This includesthe application of solder paste to the surface of a printed circuitboard or die for securing a flip chip thereto. Metal stencils arecurrently utilized to apply the solder paste onto the surface forconnecting the contact pads of surface mounted flip chips. Thesestencils typically have a plurality of apertures, each formed in thestencil in predetermined locations that correspond to the pattern of thecontact pads on the printed circuit board of choice.

In use, these stencils are positioned near or on the surface of theprinted circuit board, the apertures in the stencil are aligned over thecontact pads upon which the solder paste is to be applied, the solderpaste is then urged mechanically through the apertures via a wiper, andthe stencil is removed, leaving small islands of solder paste remainingon the contact pads of the printed circuit board.

One problem associated with the use of stencils is that an uneven, orvarying, amount of solder may be placed across the contact pads of theprinted circuit board. This adds to a lack of planarity across theprinted circuit board contact pads which may cause a subsequent reworkoperation. Further, excess solder paste can be applied that resultseither in shorting or bridging between adjacent contact pads on theprinted circuit board.

Another problem associated with the use of stencils is that the ratio ofthe height of the material to area occupied by the material is limitedby the release of the material from apertures of the stencil. Thismaterial release is a function of the cohesive forces within thematerial and the cohesive forces between the material and stencil. Asthe size of the aperture dimensions decreases, the base cross-sectionalarea of the aperture decreases; however, it is still desirable to keepthe material being applied through the aperture in the stencil at thesame vertical size or height, or greater. Further, such material appliedthrough the apertures of the stencil must be placed very close together.Unfortunately, current technology requires that as the vertical size orheight increases, the base cross-sectional area of an aperture of thestencil must increase as well for release of the material appliedthrough the apertures. This limits the pitch or spacing of the aperturesin the stencil.

One prior art solution to this problem has been to taper the wall of anaperture in the stencil so that the aperture is wider or has a largercross-sectional base area on the substrate side to provide an improvedrelease of the material from the aperture. Unfortunately, since thetaper of the aperture in the stencil is small with the aperture wallbeing substantially vertical, thereby providing a small increase in thecross-sectional base area of the aperture located adjacent thesubstrate, the material applied through the aperture can be pulled awayfrom the substrate when the stencil is removed, thus resulting in thesame problem as before.

U.S. Pat. No. 5,359,928, issued Nov. 1, 1994, discloses a screenprinting stencil that has raised edges surrounding the apertures. Theapertures also include tapered edges that provide a larger area at theportion of the stencil surface adjacent the substrate.

U.S. Pat. No. 5,460,316, issued Oct. 24, 1995, also discloses the use ofstencils and apertures with tapered walls. The apertures having taperedwalls provide a larger cross-sectional base area of the apertureadjacent the substrate than at the cross-sectional area at the openingor top of the aperture into which the solder paste is applied to thestencil. In both references, the larger cross-sectional base area of theaperture in the stencil is provided to reduce the amount of solder pastepulled away when the stencil is removed; however, the stencils require alarger cross-sectional base area for increased height or thickness ofthe solder paste being applied through the aperture to the substrate.

Accordingly, it would be advantageous to overcome the problems inherentin the prior art solutions of using stencils while retaining sufficientmaterial applied to the substrate upon removing the stencil in order tomanufacture increased height or thicknesses of material applied throughthe apertures in the stencil while facilitating a reduced pitch orspacing of the apertures in the stencil.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a stencil for use in applyingmaterial to semiconductor devices during the fabrication thereof. Thestencil of the present invention includes apertures having a portionformed by a first wall extending from a first side of the stencil and aportion formed by a second wall extending from the second side of thestencil to minimize the shear stress between the stencil and thematerial being applied to the semiconductor device. The stencil allowsmaterial to be extruded through the portion formed by the first wall ofeach aperture of the stencil to the surface of the semiconductor devicewithout contacting the wall of the second portion of the aperture of thestencil. Since the material only contacts a small area of the portionformed by the first wall of the aperture in the stencil, substantiallyall the material applied through the aperture remains on thesemiconductor device and not in the aperture of the stencil.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 depicts a schematic diagram of a stencil aperture according tothe present invention;

FIG. 2 depicts a schematic diagram of the stencil aperture of FIG. 1 asmaterial is applied;

FIG. 3 depicts a schematic diagram of the next step of applying materialto the underlying substrate through the stencil of FIG. 1;

FIG. 4 is a schematic diagram of the final step of applying material tothe substrate through the stencil of FIG. 1;

FIG. 5 is a schematic diagram of an alternative stencil aperture;

FIG. 6 is a schematic diagram of the stencil aperture of FIG. 5 afterapplying material;

FIG. 7 is a schematic diagram of yet another alternative stencil of thepresent invention formed of multiple layers;

FIG. 8 is a schematic diagram of the stencil of FIG. 7 after applyingmaterial;

FIG. 9 is a top plan view of a stencil according to the presentinvention where a plurality of apertures is displayed;

FIG. 10 is a schematic diagram of a semiconductor device attached to aprinted circuit board (PCB) according to the present invention; and

FIG. 11 is a block diagram of a computer system incorporating the PCB ofFIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram illustrating a portion of a manufacturingapparatus 10 used for applying material to the features of asemiconductor device. As illustrated, a stencil 12 is placed upon asubstrate 14. FIGS. 1-4 schematically illustrate, in diagram form, aseries of steps for applying a material to substrate 14 through thestencil 12 where the material is extruded through apertures 16 locatedin stencil 12. Corresponding elements of the apparatus 10 and stencil 12are used throughout the several views of the present invention describedherein.

Stencil 12 includes a plurality of apertures 16 formed therethrough.Stencil 12 may be formed from any suitable material, such as, forexample, stainless steel, nickel, a substrate metal plated with a secondmetal, a nickel alloy, plastics, or other resins. Each aperture 16 ofthe stencil 12 includes a first portion formed by first wall 18extending from the top surface of stencil 12 thereinto and a secondportion formed by second wall 20 extending thereinto from the bottomsurface of stencil 12 terminating in annular shoulder 19 located betweenthe first wall 18 and the second wall 20. The second wall 20 forms asecond portion of the aperture 16 through stencil 12 that is larger incross-sectional area than the cross-sectional area of the first portionof the aperture 16 formed by the first wall 18.

In FIG. 2, once the stencil 12 is placed upon substrate 14 of asemiconductor device, a material 26 is applied across the top surface ofstencil 12 via a wiper 24. The material 26 extrudes through the firstportion of aperture 16 being constrained by first wall 18 and furtherextrudes through the second portion of the aperture 16, not contactingthe second wall 20 thereof. As illustrated, material 26 contacts surface22 (FIG. 1) of substrate 14, having an area substantially the same shapeas formed by first wall 18. The extruded material 26 only contacts thefirst wall 18 of the aperture 16 of the stencil in a small area adjacentthe top or upper end of the first portion formed by first wall 18 ofaperture 16. Illustrated in FIG. 3 is the extruded material remaining onthe substrate 14 as material element 28.

Next, stencil 12 is removed from the surface of substrate 14, leavingmaterial element 28 that comprises substantially all of the material 26extruded, or printed, through aperture 16. As schematically illustratedin FIG. 4, a small amount of the material 26 remains as excess material30 at the top of the first portion formed by first wall 18 of theaperture 16 of stencil 12. Extruding material 26 through aperture 16having a larger cross-sectional base area of the second portion formedby second wall 20 helps minimize the shear stress that is presentbetween the material 26 and the walls 18 and 20 of the aperture 16 ofthe stencil 12, which is in clear contrast to the prior art where nosuch substantially larger cross-sectional base area of the aperture ofthe stencil is found.

The cross-sectional area of the second portion formed by second wall 20of aperture 16 ranges from 1.1 to 10 times the cross-sectional area ofthe first portion formed by the first wall 18 of the aperture 16. Theratio of cross-sectional area of the second portion formed by secondwall 20 to the cross-sectional area of the first portion formed by firstwall 18 of the aperture 16 should not be so large as to allow deflectionof the stencil 12 in the area of the aperture 16 when the wiper 24presses downwardly across the surface of stencil 12. Further, the use ofthe second portion formed by second wall 20 having a substantiallylarger cross-sectional area than the cross-sectional area of the firstportion formed by first wall 18 allows the extruded material formingmaterial element 28 to have a vertical height substantially equal totwice the nominal diameter of the material element 28 at the basethereof, or greater, depending upon the viscosity of the material 26 andthe slump of such material 26 after the removal of the stencil 12.

The apertures 16 formed by walls 18 and 20 may have any desired overallshape or each portion may have any desired shape, such as square,circular, oval, rectangular, other polygonal shapes or combinationsthereof. The aperture 16 and the portions thereof formed by walls 18 and20 each have a nominal diameter. The height or thickness of the materialelement 28 is typically greater than the nominal diameter thereof, butcan also be substantially the same height and nominal diameter. Theratio of vertical height to the nominal diameter of the material element28 at the base thereof ranges from 0.1 to 10. This range translates from0.001″ to 0.050″ in height and from 0.0011″ to 0.5″, in diameter. Thethickness of the stencil 12 ranges from 0.1 to 10 times the nominaldiameter of the aperture 16 adjacent the top surface of the stencil 12in forming material element 28. This range of height to nominal diameterratios is achievable only because of the ability to extrude or apply thematerial as disclosed and illustrated herein, rather than as done in theprior art methods using other stencils. What limits the ratio of theelement height versus the diameter at the base of the material element28 is the viscosity of the material 26, as well as its thixotropicindex. Thixotropic, highly viscous materials are used that have aviscosity typically ranging from 30K to 310K centipoise withapproximately 70K centipoise being preferred. The thixotropic indextypically ranges from 1.7-3.2, with approximately 2.5 being preferred.

During the formation of material elements 28, a release agent can beapplied to the under-surface of stencil 12 and/or the apertures 16 tofurther enhance the release of material 26 therefrom.

Other embodiments of stencil 12 are possible. Alternative embodimentsare illustrated in FIGS. 5-8. FIGS. 5 and 6 illustrate a stencil thathas a sloped (frustoconical) shoulder 32 located between the first wall18 and the second wall 20. The slope of the shoulder 32 can be eitheracute or obtuse, curved, or indented, or any suitable desired shape.Stencil 12 is again placed upon the surface of substrate 14. Aftermaterial 26 is extruded through aperture 16, material element 28 remainsafter removal of the stencil 12 from substrate 14.

Yet another embodiment of stencil 12 is illustrated in FIGS. 7 and 8.FIG. 7 illustrates a stencil 12 being formed in multiple layers 34, 36thereby having formed therein first and second walls 18 and 20,respectively, of an aperture 16. The first layer 34 forms the firstportion of each aperture 16 by way of first wall 18 therein. Next, asecond layer 36, applied to the bottom of first layer 34, forms thesecond portion of aperture 16 by way of second wall 20 therein. Secondlayer 36 forms second portions of aperture 16 having largercross-sectional areas than the cross-sectional areas of the firstportions by way of the nominal diameter of the cross-sectional areasformed by second walls 20 being greater than the nominal diameter of thecross-sectional areas formed by first walls 18. If desired, more thantwo layers 34, 36 may be used. FIG. 8 illustrates material element 28,formed in a similar manner as is depicted in FIGS. 1-4, using thestencil formed of multiple layers 34, 36.

Substrate 14 forming the-semiconductor device can be any type ofmaterial, such as a silicon wafer, a ceramic base or FR-4 (FlameRetardant level 4) board, which is well known to those skilled in theart. The material 26 can be any type of material that is normallyapplied to such a surface. This includes glue, polymers, photosensitiveresins, or soldering paste.

FIG. 9 depicts a top view of a stencil 12 that includes a plurality ofapertures 16. The layout of the plurality of apertures 16 can be of anydesired configuration, preferably one complementary to the configurationof the substrate 14.

The use of an aperture 16, having a larger cross-sectional area incontact with the surface of the substrate where material 26 is extrudedonto the substrate 14, offers several benefits. One benefit is thatgreater package reliability is achieved. This is because stresses in theextruded material forming material elements 28 are reduced due to theoverall height allowing for stresses to spread out, compensating formismatched coefficient of thermal expansion (CTE) between the substrate14 and the die that will be attached to substrate 14. Typically, the CTEof the die is less than the CTE of the substrate. Another benefit isthat of higher yield. Greater yield is achieved because the materialelements 28 more consistently conform to the desired critical dimensionsand are protected or untouched in portions of the apertures 16 of thestencil 12. Another benefit is that the stencil 12 can be cleaned moreeasily, as there is less material left in a portion of the aperture 16.Yet another benefit is that of higher throughput. This is achieved sincethe stencil 12 has less material 26 remaining therein for cleaning orfor redeposition. Additionally, material seepage is greatly reduced, ifnot entirely eliminated, because the material is extruded withoutexcessive pressure being applied to the stencil to cause the deflectionthereof, thereby minimizing the occurrences of bridges or shorts formedfrom excess material applied to the substrate 14. This not onlyincreases throughput, but also yield. Additionally, since the stencil 12has less material remaining on it, it can either be cleaned morefrequently with less wear or cleaned less often, thus saving a step.Further, less pressure is needed to hold the stencil 12 in place on thesubstrate 14 since the bottom perimeter of each aperture 16 is moreeffective in controlling seepage. With this reduced pressure, less wearis placed on the stencil that results in a greater life expectancy.

Those skilled in the art will appreciate that semiconductor devicesattached to a surface using the vertical elements according to thepresent invention may comprise an integrated circuit die employed forstoring or processing digital information, including, for example, aDynamic Random Access Memory (DRAM) integrated circuit device, a StaticRandom Access Memory (SRAM) integrated circuit device, a SynchronousGraphics Random Access Memory (SGRAM) integrated circuit device, aProgrammable Read-Only Memory (PROM) integrated circuit device, anElectrically Erasable PROM (EEPROM) integrated circuit device, a flashmemory device and a microprocessor device, and that the presentinvention includes such devices within its scope.

As shown in FIG. 10, a semiconductor device 110 is attached to a printedcircuit board (PCB) 112 using material elements 28 as fabricated usingthe manufacturing apparatus 10 of FIGS. 1-8. Once the material elements28 are applied to the surface of PCB 112, semiconductor device 110 isplaced on PCB 112 and the structure is then heated sufficiently so as toaffix the material elements 28 to provide a mechanical and electricalbond between PCB 112 and semiconductor device 110. Also, as shown inFIG. 11, an electronic system 114 includes an input device 116 and anoutput device 118 coupled to a processor device 120 which, in turn, ismounted to PCB 112 having semiconductor device 110. Processor device 120can be mounted in a like manner as semiconductor device 110.

Although the present invention has been described with reference to aparticular embodiment, the invention is not limited to this describedembodiment. The invention is limited only by the appended claims, whichinclude within their scope all equivalent devices or methods whichoperate according to the principles of the invention as described.

1. A method for forming paste on a surface of an object comprising:providing a stencil having a plurality of apertures formed therethroughfrom a top surface of said stencil to a bottom surface of said stencil,at least one aperture of said plurality of apertures including a firstportion having a first cross-sectional area formed by a first wallportion having a first diameter and extending generally vertically fromsaid top surface of said stencil, a second portion of said at least oneaperture of said plurality of apertures adjacent said bottom surface ofsaid stencil and having a second cross-sectional area formed by a secondwall portion having a second diameter larger than said first diameter ofsaid first wall portion and extending generally vertically from saidbottom surface of said stencil, said stencil having a thickness in arange of from 0.1 to 10 times said first diameter of said first wallportion of said at least one aperture of said plurality of apertures,and at least one sloped annular shoulder having a shape located betweensaid first wall portion and said second wall portion of said at leastone aperture of said plurality of apertures; applying said stencil tosaid surface of said object; applying paste to said stencil; wiping saidpaste across said top surface of said stencil to force said pastethrough said plurality of apertures; preventing contact of said pastewith a portion of said second wall portion of said at least one apertureof said plurality of apertures during said applying said paste to saidstencil by said second cross-sectional area of said second portion ofsaid at least one aperture of said plurality of apertures being largerthan said first cross-sectional area of said first portion of said atleast one aperture of said plurality of apertures; and removing saidstencil and leaving portions of said paste in a substantially verticalcolumn.
 2. The method according to claim 1, wherein said applyingfurther comprises: applying said paste to said top surface.
 3. Themethod according to claim 1, wherein said paste has a viscosity ofapproximately 70K centipoise.
 4. The method according to claim 1,wherein said paste has a thixotropic index ranging between about 1.7 to3.2.
 5. The method according to claim 1, wherein said paste has athixotropic index of approximately 2.5.
 6. The method according to claim1, wherein said stencil is made of stainless steel.
 7. The methodaccording to claim 1, wherein said stencil is made of plastic.
 8. Themethod according to claim 1, wherein said at least one sloped annularshoulder slopes from said first portion of said at least one aperture ofsaid plurality of apertures towards said second portion of said at leastone aperture of said plurality of apertures.
 9. The method according toclaim 8, wherein said at least one sloped annular shoulder has an acuteshape.
 10. The method according to claim 8, wherein said at least onesloped annular shoulder has an obtuse shape.
 11. The method according toclaim 8, wherein said at least one sloped annular shoulder has anindented shape.